Encapsulant systems made of glass are widely used, in particular in the electronics industry. One major aspect of their functions involves the protection of the electronic components from the environment. For example, the encapsulation of conductor metals protects them from long-term corrosion, while hybrid circuits should be encapsulated to insure resistor stability in a humid atmosphere.
A glass composition suitable for encapsulant systems must exhibit a wide range of desired physical and chemical features, such as low melting point, limited thermal expansion, appropriate flowability, and good resistance to acid or base attack. However, as will now be explained, it is difficult to group together all of the properties indicated above.
For example, in order to protect the electrical components from the environment, the encapsulant should have sufficient durability to survive the environments encountered in the production and the daily use of the electronic circuits. Most glasses having a low softening point are characterized by poor durability in acids and bases, and their durability tends to degrade as the glass transition temperature (Tg) becomes lower. Although the majority of electronic circuits are not expected to be used in very acidic or basic environments, some are exposed to water and basic or acidic environments during the production. Furthermore, since the final stage in some fabrication processes involves an additional encapsulation by an organic polymer, e.g., an epoxy, wherein some of these resins contain an amine that can exert basic environment in a humid atmosphere, it is important that the system be able to withstand said conditions, as well.
Another difficulty in formulating a suitable glass composition for encapsulation systems stems from the fact that various glasses having low glass transition temperature (Tg) are usually characterized by a high temperature coefficient of expansion, herein designated as a, which, unless it is carefully matched to the adjacent circuit layers, can set up substantial mechanical stresses which can lead to system failure.
The degree of flowability which is required from the glass at the firing temperature is yet another feature which must be carefully controlled. The encapsulant must form a bubble-free seal at a low enough firing temperature without interacting with the underlying electrical components. If the glass flows too much, it will diffuse into the electrical components, thereby altering its electrical characteristics. If it does not flow enough, it will not seal. The organic vehicle necessary for the screen printing operation must burn out at this temperature. Thus, an ideal encapsulant should screen print smoothly and rapidly with a vehicle which is decomposable at a temperature low enough to allow the glass to flow sufficiently to form a seal, but not so much as to alter the resistor electrical characteristics.
The prior art countered these problems in various ways. The introduction of cadmium and lead to the glass composition provides important characteristics such as moderate linear coefficient of expansion, low viscosity at fairly low temperatures, improved resistance and durability compared to glasses containing high concentrations of alkali oxides. A glass composition comprising PbO is illustrated, for example, in U.S. Pat. No. 5,114,885 to Hormadaly. However, adverse toxicological effects attributed to cadmium and lead have resulted in various restrictions in the use of such glass compositions, despite their excellent properties.
The art has also attempted to provide cadmium-free and lead-free glass compositions, but these compositions suffer from various drawbacks and do not provide fully satisfactory properties.
U.S. Pat. No. 4,613,539 and U.S. Pat. No. 4,707,346 to Hormadaly, and U.S. Pat. No. 4,966,926 to Donhaue, disclose conductive phases such as LaBr.sub.6, and compositions of tantalum and/or niobium doped tin oxide conductive materials. These materials contain Cd and Pb-free glasses, and require processing in an inert atmosphere. They cannot be processed in air, because the conductive phase will oxidize. The oxidation in air renders these conductive phases insulators (LaB.sub.6) and inappropriate (doped SnO.sub.2) for resistor use, due to intrinsic change in electrical properties. In addition, these patents disclose glass compositions with high melting points, which prevents them from being used in low temperature overglaze applications. U.S. Pat. No. 4,892,847 to Reinherz discloses glass compositions containing Bi.sub.2 O.sub.3 as the major ingredient for vitreous coating applications. The compositions according to Reinherz have critical compositional limits for the SiO.sub.2, Bi.sub.2 O.sub.3, B.sub.2 O.sub.3 and the alkali oxides ingredients, the concentration of said Bi.sub.2 O.sub.3 typically being below 10 mole % and the concentration of said B.sub.2 O.sub.3 typically being above 15 mole %. They are alleged to exhibit a variety of desirable properties. However, their performance characteristics are still not satisfactory. For example, the reported thermal coefficients of expansion for these glasses, being in the relatively high range between 70 to 90.multidot.10.sup.-7 /.degree. C., diminish their applicability to thick film microelectronics. The durability of the glasses according to U.S. Pat. No. 4,892,847, which has been tested only in mild conditions (citric acid and commercial dishwasher detergent), was found to be below the required degree to withstand the attack of 0.1N HCl and 0.1N NaOH solutions.
U.S. Pat. Nos. 5,439,852 and 5,491,118 to Hormadaly disclose Bi.sub.2 O.sub.3 glasses with a broad concentration range of 5-70 mole %. However, it has been found that these glasses lack sufficient durability in acidic and basic environments. In addition, it has been found that the thermal expansion of these glasses is too high for the overglaze applications.
U.S. Pat. No. 4,970,178 to Klimas et al. discloses glass compositions for use in vitreous coatings consisting essentially of SiO.sub.2, B.sub.2 O.sub.3 and Na.sub.2 O, Bi.sub.2 O.sub.3 being an optional additional ingredient in an amount not higher than 10 mole %. The durability of the compositions according to U.S. Pat. No. 4,970,178 was tested only with respect to an acid attack, the acid being 4% acetic acid.
It is therefore an object of the present invention to provide a glass composition which is cadmium-free and lead-free, having sufficient durability in acidic and basic environments.
It is another object to provide thick film formulations comprising the glass compositions of the present invention for various applications.
Additional objects and advantages of the present invention will become apparent as the description proceeds.